How do I make my code so it only spawns one object in the area i chose?

Can someone please help me figure out how to make it so when I click play it only spawns one object because when I click play it spawns a ton of objects that fill up the space. please help.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SpawnObject : MonoBehaviour {

public Vector3 center;
public Vector3 size;
public GameObject KeyPrefab;

// Use this for initialization
void Start () {

// Update is called once per frame
void Update () {

public void SpawnKey()

    Vector3 pos = center + new Vector3(Random.Range(-size.x / 2, size.x /2), Random.Range(-size.y / 2, size.y / 2), Random.Range(-size.z / 2, size.z / 2));

    Instantiate(KeyPrefab, pos, Quaternion.identity);


void OnDrawGizmosSelected() 
    Gizmos.color = new Color(1,0,0,0.5f);
    Gizmos.DrawCube(center, size);


Is the script that you are using connected to your prefab? If so, when a new GameObject is created, it runs your Start() function and creates another clone of itself and continues the infinite chain of creations. I would suggest creating an Empty GameObject with merely that script attached.